Platelet accumulation at sites of vascular injury is a dynamic process that mediates formation of both the primary hemostatic plug and pathologic thrombus formation. The mechanisms by which platelet surface proteins direct platelet recruitment to thrombi under flow conditions have been studied in detail. In addition to directing initial platelet adhesion, cell-surface receptor interactions activate intracellular signaling. Intracellular signaling stimulates the release of thrombogenic substances from platelet granules. Signaling also mediates activation of the platelet integrin αIIbβ3 that facilitates firm adhesion of platelets to sites of injury.
Arterial thrombosis mediates tissue infarction in coronary artery disease, cerebrovascular disease, and peripheral vascular disease, and, thus, is the single most common cause of morbidity and mortality in the United States. Platelets are key mediators of arterial thrombosis. Thus, the identification of compounds that inhibit platelet function is of great importance to medicine.
Platelets form the body's primary means of hemostasis and, as such, have developed an elaborate mechanism of surveying the vasculature for defects in endothelial integrity. This mechanism involves the ability to respond to subendothelial matrices, shear forces, neighboring platelets, the adrenal axis, as well as soluble proteinacious, nucleotide, and lipid signals. Despite this plethora of physiologic activators, the platelet has only a small repertoire of major functional outputs. Upon activation, platelets change shape, aggregate, and secrete their granular contents. The process of platelet activation involves the expression of activities not shared by functionally intact resting platelets, including, for example, ATP release, serotonin release, lysosomal release, alpha granule release, dense granule release, and cell surface expression of markers of activated platelets (including, but not limited to P-selectin and activated GPIIb/IIIa receptor). In addition, platelet activation results in the aggregation of platelets with each other and attachment to non-platelet surrounding cells. The granular contents of platelets supply additional adhesion molecules, growth factors, coagulation enzymes and other specialized molecules instrumental in the process of thrombus formation and the initiation of the healing process.
In addition to coronary artery disease/myocardial infarction, cerebrovascular disease and peripheral vascular disease, diseases and disorders associated with inappropriate platelet activity and arterial thrombosis also include, for example, stable and unstable angina, transient ischemic attacks, placental insufficiency, unwanted thromboses subsequent to surgical procedures (e.g., aortocoronary bypass surgery, angioplasty and stent placement, and heart valve replacement), or thromboses subsequent to atrial fibrillation. Inhibitors of platelet activity can provide therapeutic and preventive benefits for each of these diseases or disorders. It is also possible that inappropriate platelet activation plays a role in venous thrombosis, such that platelet inhibitors can be useful for the treatment or prophylaxis of disorders associated with such thromboses.
A connection is emerging between platelet activation and inflammation, particularly allergic inflammation (e.g., in asthma) and inflammation at the sites of atherosclerotic damage. Therefore, compounds that inhibit platelet activation can also be useful in the treatment or prophylaxis of disorders involving inflammation.
There are a number of agents presently available that target platelet function. For example, aspirin is a relatively weak platelet inhibitor. However, aspirin can cause life-threatening allergic reactions in sensitive individuals.
Another platelet inhibiting agent is ticlopidine (Ticlid™, Roche Pharmaceuticals). Because it requires the production of active metabolites to be effective, the effect of ticlopidine is delayed 24-48 hours. The drug can also cause thrombotic thrombocytopenic purpura as well as life threatening leukopenia, nausea, abdominal pain, dyspepsia, diarrhea and skin rash.
Clopidogrel (Plavix™, Bristol-Meyers Squibb/Sanofi Pharmaceuticals) is another platelet inhibitor that requires the generation of active metabolites for its therapeutic efficacy. Therefore, clopidogrel also has a delay of 24-48 hours for its effect. Clopidogrel can also cause thrombotic thrombocytopenia purpura. The drug has also been associated with rash, edema, hypertension, hypercholesterolemia, nausea, abdominal pain, dyspepsia, diarrhea, urinary tract infections, liver enzyme elevations and arthralgia.
The platelet inhibitory agent abciximab (c7E3 Fab, Reopro®, manufacturer-Centocor B. V., distributor-Eli Lilly and Co.) is only available in a parenteral form. The drug can cause severe thrombocytopenia. Its antiplatelet effects last for several days and, therefore, may complicate surgery that is sometimes required in the setting of life-threatening arterial occlusion (e.g., emergent cardiac surgery in the setting of a myocardial infarction).
Tirofiban (Aggrastat™, Merck and Co., Inc.) is another platelet inhibitory agent that is only available in a parenteral form. Tirofiban can cause thrombocytopenia, dizziness and vasovagal reactions.
Eptifibatide (Integrilin™, COR Therapeutics, Inc., Key Pharmaceuticals Inc.) is another platelet inhibitory agent that is only available for parenteral administration. It can cause thrombocytopenia and hypotension.
There is only limited clinical experience with the oral anti-GPIIbIIIa agents, lamifiban, sibrafiban, orbofiban and xemilofiban. Similarly, clinical experience is limited with the phosphodiesterase inhibitors cilostazol, trapidil and trifusal. There is more clinical experience with the phosphodiesterase inhibitor dipyridamole, but its activity is so weak that it is not frequently used.
There is a need in the art for additional platelet adhesion and aggregation inhibitory agents for the treatment and prophylaxis of diseases or disorders associated with abnormalities in platelet adhesion and aggregation.
It is known that integrin αIIbβ3 is a receptor on the surface of human platelets. As a heterodimeric complex composed of both αIIb and β3 subunits, the dimer is responsible for binding adhesive plasma proteins, most notably fibrinogen and von Willebrand factor (vWF). The binding of fibrinogen, vWF and other ligands by αIIbβ3 is mediated principally though the peptide recognition sequence Arg-Gly-Asp (RGD) or the dodecapeptide HHLGGAKQAGDV. Currently two small molecule inhibitors of the αIIbβ3 exist: a cyclic homoarginine-glycine-aspartic acid peptide (eptifibatide) and an
RGD peptidomimetic (tirofiban). Both inhibitors act by competitively blocking the binding site for fibrinogen.
Conformational changes in αIIbβ3 are thought to occur upon the binding of ligand to the receptor, leading to the exposure of ligand-induced binding sites (LIBS) as detected by LIBS-specific monoclonal antibodies (mAbs). Electron microscopy and crystal structures of the integrin in complex with various R(K)GD-like ligands support the theory that the integrin undergoes a major conformational change after or during ligand binding. It is thought that the binding of the existing small molecule αIIbβ3 inhibitors induces these conformational changes in the integrin's extracellular structure. Upon dissociation of these ligands, the integrin is thought to remain in its high affinity conformation, capable of binding circulating soluble fibrinogen, thus increasing the risk for thrombotic complications following αIIbβ3 inhibitor therapy.